Process for producing cold-rolled steel plates high in the cold-formability

ABSTRACT

PROCESS FOR PRODUCING COLD-ROLLED STEEL PLATE HAVING GOODSURFACE PROPERTIES AND EXCELLENT COLD-FORMABILITY FROM A RIMMED STEEL OR CAPPED STEEL CONSISTING OF LESS THAN 0.10%C, LESS THAN 0.4% MN AND LESS THAN 0.02% S, THE RATIO OF MN TO S BEING MORE THAN 10, AND THE REST BEING IRON AND IMPURITIES BY SUBJECTING AN INGOT TO HOTROLLING WITH A TEMPERATURE AT THE FINISH WHICH IS ABOVE THE AR3, TRANSFORMATION POINT, COILING AT A TEMPERATURE BELOW 570*C., AND COLD-ROLLIG, AND FINALLY TO RECRYSTALLIZATION ANNEALING BY HEATING IN A NON-DECARBURIZING ATMOSPHERE AT AN INCREASING TEMPERATUREE AT A RATE OF FROM 5-50* C./HR.

United States Patent Ser. No. 330,651

Claims priority, appgc/atiog Japan, Mar. 2, 1968,

Int. Cl. mid 25/06 US. Cl. 148-2 Claims ABSTRACT OF THE DISCLOSUREProcess for producing cold-rolled steel plate having good surfaceproperties and excellent cold-formabihty from a rimmed steel or cappedsteel consisting of less than 0.10% C, less than 0.4% Mn and less than0.02% S, the ratio of Mn to S being more than 10, and the rest beingiron and impurities by subjecting an ingot to hotrolling with atemperature at the finish which is above the Ar transformation point,coiling at a temperature below 570 C., and cold-rolling, and finally torecrystallization annealing by heating in a non-decarburizing atmosphereat an increasing temperature at a rate of from 550 C./hr.

This application is a continuationdn-part of application Ser. No.803,611, filed Mar. 3, 1969, now abandoned.

This invention relates to a process for producing coldrolled steelplates having few surface flaws and excellent cold-formability(deep-drawability and extrudability in the present invention).

In forming a cold-rolled steel plate by a pressing operation, it isrequired that the steel plate should have good deep-drawability andextrudability depending on the use.

In making an ingot from a molten steel when producing a cold-rolledsteel plate, the steel is metallurgically largely classified into akilled steel, semi-killed steel or rimmed steel according to itsdeoxidized state. The killed steel is a steel which has been fullydeoxidized and has excellent properties, but it has a drawback that itiscostly. 0n the other hand, rimmed steel has much oxygen in the moltensteel so that it may combine with C simultaneously present in the moltensteel and a rimming action may be caused by CO gas evolved in thev mold.Rimmed steel is inferior in its properties to killed steel, butis socheap that it is used in large quantities. H

Further, steel in which a part of the rimming action is mechanically orchemicallycontrolled is called capped steel, which is close to rimmedsteel in its properties and price. In order to increase the rimmingaction and to obtain a sound rim layer, it is considered desirable that0.07 to 0.10% C be contained in the molten steel.,On the other hand, Cis an element detrimental to the coldformability of the product and fromthis viewpoint it is desirable that it be lower.

However, if C in the molten steel is reduced to less than 0.07%, therimming action is weak, and many small bubbles remain in the ingotwhereby the soundness of the rim layer is greatly reduced. These bubblesdistributed near the skin of the ingot are subjected to oxidation duringthe subsequent soaking heating, and can not be pressed out, causingthereby the formation of surface flaws in the rolled product.

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The present invention seeks to solve these contradicting problems of thesurface properties and cold-formability at one stroke and has as anobject to provide a steel plate in which both of these properties areexcellent.

Other objects of the present invention will be clear from the followingdescription and accompanying drawing.

In the accompanying drawing:

FIG. 1 shows the influences of the absolute amounts and combinations ofvarious Mn and S contents in a rimmed steel and capped steel on the Fvalue and n value of the final product.

The present invention will be explained more particularly in thefollowing.

The present inventors have discovered that for a steel which hasa Cladle content less than 0.07% so that the product formed therefrom hasgood cold-formability, but which steel has insufficient rimming action,resulting in inferior surface properties of the product, the bubblesnear the ingot skin'can be greatly reduced, the problem of the surfaceflaws in the product can be eliminated and the cold-formability can befurther greatly improved. This can be done by reducing the MN in saidsteel to a value so low as to be inconceivable from the standpoint ofcommon sense in any conventional cold-rolled steel plate, that is, lessthan 0.02%. With regard to a steel which has a high C ladle content suchas 0.07 to 0.10% and therefore has a sufficient rimming action, andconsequently no problem exists with respect to the surface properties,but which steel is inferior with respect to cold-forrnability, thecold-formability of such steel can be made far superior to anyconventional product of today by regulating the Mn and S contents sothat they are within the hereinafter described range and by processingthe steel, i.e. hot rolling, cold rolling, coiling and annealing underthe proper conditions.

The deep-drawability and extrudability of a cold-rolled steel plate arerepresented usually respectively by an F value and an n value.

The F value, which is also called the plastic strain ratio, is amechanical characteristic represented by the ratio of the width strainto the thickness strain of a tension test piece and is known to show avery favorable correlation with the deep-drawability of a metal plate.The F value is a mean value or F value in each direction in the plane ofthe plate.

The larger the F value, the higher the deep-drawability. It has a valueof about 1.3 in an ordinary rimmed steel.

The n value so called here is also called a work hardening index and isknown to have a good correlation with the extrudability of the metalplate. The n value corresponds to the index n when the true stress(u)-stra.in (e) curveobtained by the tension test of the material isapproximately 0=Ce (wherein C is a constant) and is generally determinedby the average gradient of the log. -log. 2 curve in the strain range of10 to 20%. The larger this value, the higher the extrudability. It isabout 0.22 in an ordinary rimmed steel.

FIG. 1 shows the relations between the F value and n value for the Mnand S contents of products manufactured from 43 kinds of ingots ofrimmed steel and capped steel, said ingots having combinations of the Mncontent and S content within the ranges of 0.02 to 0.40% and 0.003 to0.030% respectively, with chemical components other than Mn and Sremaining unchanged. The products were made subjecting said ingots tothe following steps: blooming, hot-rolling, cold-rolling at a reductionrate of 70%, coiling, and lastly to recrystallization annealing in aneutral atmosphere at 700 C. for 4 hours. It is clearly recognized fromthis graph that the F value and n value are influenced by both Mn and S.In the graph, the value and the n value are represented respectively byround and square marks on the same material. However, in this graph, theposition of'the round'mark indicates the right position. The square markdrawn to show the n value is placed beside the corresponding round mark.

On' the other hand, as described later, unless Mnis present in an amountofmore than times'S, the material will breakor crack due to thephenomenon of redhot brittleness during the hot-rolling.

The i value of the materials on the low 'Mn side in FIG. 1 is thatconverted from a measured value for a final product obtained fromamaterial partly 'cracked in 'l'hot rollin g' as a mother material,saidfmeasu'red value being obtained by quantitatively measuring'thecrystal sur face parallel with the plate plane. of the final product byX-rays. Because the sample to be used'for the X '-"'ray meas- .',u'rement is large" enough even though it is smaller than that needed for amechanicaltest.even'apartly cracked material can be used as'a sample,while avoiding the icracked part. Further, as is described in detail,for instance, in the Japan Metal Society'Iournal, Vol.29, No.4,

given for the sample broken by 0.08 to 0.18%) and for an S content arange. of 0.004 to 0.020% (preferably 0.008 to 0.018% should be selectedif an i value exceeding 1.3, which is an approximate average level ofthe F value for a rimmed steel or for a capped steel of a rimmed steelor for a capped steel, and an n value of more than 0.22, which is anapproximate average level of the n value of the same, are set astargetsto be attained. When Mn and S were contained in these ranges, and theconditions under which the steel was made were as described hereinafter,an F value of 1.3 to 2.0 and nvalues of 0.26 to 0.32 were obtained. Atpresent, conventional steels of the same kind contain Mn and S in theranges of about 0.30 to 0.40% and 0.010 to 0.025% respectively and havean 7 value of 1.1 to 1.4 and an n value of 0.20 to 0.24, which clearlydemonstrates the superiority of the present method, when compared withthe above-mentioned values of the steel of the present invention.

On the other hand, in metallurgy there is a phenomenon called red-hotembrittlement, in which embrittlement of the steel is caused when it ishot-rolled. This is considered to be caused by the fact that S in thesteel is reticulately deposited as FeS around an initial crystal. As acountermeasure for preventing this phenomenon there has been widelyadopted a practice of adding Mn in an amount matching the S.

Because of the segregation of S in an ingot, there has been heretoforeempirically carried out the step of adding Mn in an amount such that theratio of Mn to S210 in the case of a rimmed steel or a capped steell'However, if this relation should be applied to the optimum range in FIG..1, the ratio of Mn, to'S will be within'the part hatched with diagonallines. In fact, it is evident from this'graph that any sample containingMn and S Withinthis' hatched range has a very favorablecold-formability. The range hatched with diagonal lines shall be calledthe designated range of FIG. 1 hereinafter.

C in an amount of 0.l0% is adopted as a ladle composition of anordinaryrimmed steel or capped steel. It has already been attempted byvarious means to improve the cold-formability by adding a specialelement to an ordinary rimmed steel. However, according to the resultsof the research of the present inventors, unless a special producingstep is taken, the additionof V and B must, on'the contrary, reduce the7 value. Therefore, if the steel of the present invention is to be madenonageable, these elements must not be added. Further, in order toimprove the workability, the recrystallizing annealing is often carriedout in a decarburizing atmosphere. However,

in the steel of the present invention, it is possible to obtain a verygood workability even without carrying out such a specialtreatment. Thisis very advantageous to the producing cost.

A method of carrying out the present invention shall be described in thefollowing.

A molten steel made in a converter or open-hearth furnace and having aladle composition of C0.l0% and Mn and S within the designated range ofFIG. 1 is top-poured or bottom-poured so as to be a rimmed steel. Inthis case, it may be made a capped steel by mechanically and chemicallycontrolling the rimming action. A coldrolled steel plate is made fromthe thus obtained ingot through respective blooming, hot-rolling, coldrolling and recrystallizing annealing steps by a conventional process.The steel is hot-rolled at a temperature such that the temperature atthe finish of the hot-rolling is above the Ar point, and is cold-rolledat a reduction rate in a range of 50 to By keeping the finishingtemperature for the hot rolling above the Ar;, transformation point, thedevelopment of crystals of steel which have random crystal orientationand have a ()-orientation harmful to deep drawability is prevented.

The steel must then be cooled to below 570 C. and coiled whereby Mn andS contained in the steel can be held in solid solution, and they areprevented from being precipitated. The cooling can be by conventionalwater spray means. Then the steel which has been thus hotrolled, cooledand-coiled, is cold-rolled and then subjected to a recrystallizationannealing. In the annealing step, the steel is slowly heated at aheating rate of 5-50 C./hr. in a non-decarburizing atmosphere. Theheating must be to a temperature above 500 to 550 C., and should be nohigher than the Ar transformation point, about 860 C. The higher thetemperature within this range, the better for the formation of a (111)texture. If the steel is left in the coiled condition for the annealing,it is preferred that the upper limit of the temperature not exceed 7500., since sticking may occur in the coil. The most preferred temperatureis 700 to 710 C. The time at the upper limit should be at least 10minutes, but if subsequent grain growth is desired, a longer time shouldbe used. Usually 4 hours gives a satisfactory result.

Applicants have found that when the steel contains the components and issubjected to the sequence of treating steps under the conditionsdescribed above, the manganese and sulfur compound which is precipitatedis predominantly B-Mn/S in finely divided form. Because the Mn/S ispredominantly present in the [3 form, the collective structure is givena (111)-orientation, which is desirable for enhancing the deepdrawability of the steel. Some a-Mn/S will be precipitated, but inamounts which are insufiicient to adversely affect the properties of thesteel as compared with the effect produced by the amount of 5-Mn/S.-

Applicants have further found that the fl-Mn/S, which has a zinc blendore type crystal structure closely resembling wurtzite ore typestructure, will be precipitated in inclusions which have a size below0.5, and that these are combined with crystals of the metal intocrystals having a (111)-orientation which contribute to the improvementof the 7- value of the steel, whereby an 7 value of more than 1.8 to1.9can be obtained.

When components are present in amounts other than as described above andthe steps are not carried out as described, a type Mn/ S isprecipitated, which has a rock salt type structure which is detrimentalto the 7 value.

EXAMPLE 13 tons of a molten steel of 0.07% C, 0.12% Mn and 0.009% S madein a converter were top-poured into a downwardly expanding flat mold toproduce a rimmed or capped ingot. This ingot was hot-rolled, was thencoldrolled with a reduction rate of 70% and was annealed in a neutralatmosphere at 700 C. for 4 hours to form a cold-rolled steel plate 0.8mm. thick. Its chemical composition was as follows: for the rimmedsteel, 0.04% C, 0.11% Mn, 0.007% Si, 0.009% S and 0.013% P, the restbeing Fe and unavoidable impurities, and for the capped steel, 0.051% Cand the others substantially the same as in the rimmed steel. Thesurface tests results and cold-formability in this case as compared withthose in steel with a conventional composition and made by aconventionalmethod are shown in the table.

The conventional composition and method here designates a treatment of arimmed steel ingot containing 0.07% C, 0.33% Mn and 0.018% S by the sameprocess as is described above. By the following comparison the effect ofthe method of the present invention can be clearly demonstrated.

(In the method of the present invention, the upper line shows theresults of the rimmed steel and the lower line shows the results of thecapped steel.)

What is claimed is:

1. A process for producing cold-rolled steel plates having excellentcold formability, comprising the steps of ingoting a molten steel madein a converter or openhearth furnace and having a composition of C0.10%,less than 0.2% Mn and less than 0.02% S, the ratio of Mn to S beinggreater than 10, and the balance iron and impurities, as a rimmed steelor capped steel, hotrolling the ingot, cooling the hot-rolled steel to atemperature no higher than 570 C. and coiling the steel, coldrolling thesteel at a reduction rate of about 70% and then subjecting thecold-rolled steel to a recrystallization an-- nealing by heating thesteel at a rate of 5-50 C./hr. up to a temperature in a range of fromabout 500 C. to the Ar;; point for a period of from 10 min. to 4 hours.

2. A process as claimed in claim 1 wherein said molten steel has C in anamount of 0.10 to 0.070%, Mn in an amount of 0.04 to 0.20%, and S in anamount of 0.004 to 0.020%, and the balance being Fe and impurities.

3. A process as claimed in claim 1 wherein said molten steel has C in anamount less than 0.070%, Mn in an amount of 0.04 to 0.20%, and S is anamount of 0.004 to 0.020%, and the balance Fe and impurities.

4. A process as claimed in claim 1 wherein said recrystallization stepcomprises heating the steel to 700 C. for about 4 hours.

5. Cold rolled steel having excellent cold formability, said steel beingproduced by the process comprising the steps of ingoting a molten steelmade in a converter or open-hearth furnace and having a composition ofC0.10%, less than 0.2% Mn and less than 0.02% S, the ratio of Mn to Sbeing greater than 10%, and the balance iron and impurities, as a rimmedsteel or capped steel, hot-rolling the ingot, cooling the hot-rolledsteel to a temperature no higher than 570 C. and coiling the steel,cold-rolling the steel at a reduction rate of about and then subjectingthe cold-rolled steel to a recrystallization annealing by heating thesteel at a rate of 5-50" C./hr. up to a temperature in a range of fromabout 500 C. to the Ar point for a period of from 10 min. to 4 hours.

References Cited UNITED STATES PATENTS 3,668,016 6/1972 Shimizu et al148-2 2,878,151 3/1959 Beall et al. 14812 RICHARD O. DEAN, PrimaryExaminer US. Cl. X.R.

